Process for kraft chemical recovery

ABSTRACT

A method of recovering chemicals from spent black liquor used in a kraft pulping process comprising organic materials having oxygen containing functional groups, the method comprising heating the black liquor at an elevated temperature and at a corresponding pressure at which there is no or substantially no boiling of the black liquor, until all or a portion of the oxygen is converted into carbon dioxide to form a slurry comprising microparticulate char and a green liquor suspension. The heating can be accomplished with any suitable means. For example, black liquor is evaporated in an evaporator train, and the liquor pumped in a high pressure pump to a suitable pressure, as for example, a pressure of from about 1200 psig to about 2000 psig. Thereafter, the liquor is transferred to a heat exchanger and heated to the elevated temperature, as for example a temperature of at least about 475° F. The liquor is then transferred to a reactor and is maintained in the reactor until the spent black liquor is reduced to a slurry.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a process for recovering kraft chemicals fromspent cooking liquors in the paper pulping process. In particular, theinvention relates to a novel way of processing the liquors as a slurryrather than as a homogeneous solution.

2. The Prior Art

In a typical papermaking process, pulp is prepared from wood by eitherchemical or mechanical pulping processes. In the more common chemicalprocess, wood chips are loaded into a pressure vessel known as adigester which is charged with a chemical reagent. Heat is supplied tothe digester to "cook" the wood chips to remove undesirable substancessuch as lignin from the wood and to liberate the desirable cellulosicfiber.

A widely used chemical pulping process is the "kraft" pulping process,which uses an alkaline chemical reagent referred to as "white liquor" toact upon the lignin in the material. Typically, the white liquor is analkaline solution of sodium hydroxide (NaOH) and sodium sulfide (Na₂ S),provided by an aqueous solution typically containing between about 80-90grams/liter of NaOH and about 20-25 grams/liter Na₂ S with minor amountsof sodium carbonate, sulfate and thiosulfate. Depending upon the woodspecies used and the desired end product to be manufactured, whiteliquor is added to the wood chips in sufficient quantity to provide atotal charge of alkali of 15-20% Na₂ O based on the dried weight of thewood.

Typically, the temperature of the wood/liquor mixture is maintained atabout 165°-170° C. for a total reaction time of about 2-3 hours. Whendigestion is complete, the pressure in the digester is released and theresulting wood pulp is separated from the spent liquor, called "blackliquor" in a series of washing operations.

The pulping chemicals are recovered from the black liquor by a processthat is commonly referred to as the recovery process. The black liquor,which contains organic constituents and sodium, is concentrated and thenburned in a process referred to as smelting to reduce the organicconstituents to CO₂ and H₂ O. The ash from the smelting operationcontaining sodium carbonate, or soda ash, is mixed with water to form aweak solution known as "green liquor". The solid particles in the greenliquor, the "dregs" are then removed from the green liquor. The greenliquor is subsequently causticized through the addition of lime toregenerate white liquor. This entire process can be time consuming andexpensive.

It has been known that low grade fuels such as industrial waste may bemore efficiently converted to energy if the fuel is provided in a slurryand heated at high pressure until the chemically bound oxygen isconverted to CO₂ gas. Such a process is described in U.S. Pat. No.5,485,728 to Dickenson.

Thermal treatment thus appears to have beneficial properties in theprocessing of waste materials. Its application to the chemical recoveryin paper pulping processes has not been done prior to the presentinvention but would appear to offer beneficial results.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide animproved method for the recovery of the kraft chemicals using fewersteps than the conventional method.

It is anther object of the present invention to provide an improvedmethod for kraft chemical recovery that reduces black liquor solids,water and liquor volume.

It is a further object of the present invention to provide an improvedmethod for kraft chemical recovery that increases the capacity of theexisting recovery process.

It is yet a further object of the invention to provide an improvedmethod for kraft chemical recovery that is less expensive thantraditional methods.

These and other objects of the invention are accomplished by a methodfor recovering kraft chemicals comprising heating black liquor used inthe kraft pulping process and comprising organic materials having oxygencontaining functional groups for a sufficient time and at a sufficientelevated temperature and corresponding pressure (such that there is noor substantially all boiling) to convert all or a portion of the mass ofoxygen into carbon dioxide. The elevated temperature may vary widely andis a temperature which is sufficiently high to carry out the desiredconversion of oxygen to carbon dioxide. The upper temperature limitedmay be of any value and is primarily limited by the capability of thereaction or heating vessel or apparatus to withstand the resultingpressure. The temperature is preferably at least about 475° F., morepreferably from about 475° F. to about 625° F. and most preferably fromabout 500° F. to about 575° F. In the embodiments of choice, thetemperature is about 550° F. Heating times may very widely. The blackliquor is heated at the elevated temperature for a time sufficient toconvert the desired amount of oxygen into carbon dioxide. In general,the higher the temperature, the shorter the heating time required toconvert a given mass of oxygen into carbon dioxide; and conversely, thelower the temperature, the longer the heating time required for suchconversions. Usually, when employing preferred temperatures, heatingtimes may be from 0 to about 60 minutes or longer converted from ahomogeneous alkali lignin/soda-sulfur/water solution into amicroparticulate char slurry in a green liquor suspension.

The reaction chemistry of this process is as follows:

    Na.sub.2 CO.sub.3 →Na.sub.2 CO.sub.3

    Na.sub.2 SO.sub.4 →Na.sub.2 S+20.sub.2

    Alkali Lignin→Char+NaOH+Na.sub.2 S+Unknowns

In the reducing environment (absence of O₂) of the thermal treatment,salt cake is reduced to sodium sulfide in a similar manner to thereduction in the smelt bed of a recovery boiler. Sodium carbonate, as ina recovery boiler, is retained as the same. Alkali lignins are reducedto carbonaceous char, sodium hydroxide, sodium sulfide and other unknownsoda compounds.

During this process, the following two characteristics are observed:

1) All or a portion of the oxygen containing functional groups such asC--O, C═O and O--C═O terminal and conjugate groups contained in theorganic material component of the spent black liquor are split off asCO₂, thus decreasing the degree of polymerization and thus the averagemolecular weights of components such as alkali lignins and carboxylicacids.

2) All or a portion of the oxygen present in the organic materialcomponent of the spent black liquor is converted to carbon dioxide inthe reaction. The actual amount of oxygen converted will vary dependingupon a number of factors as for example, the types of organic componentsin the black liquor as for example resins and acids. In general, higherpercent conversions of oxygen are preferred. Usually, at least about 50%of the total mass of oxygen is converted into carbon dioxide. Thepercent conversion is preferably at least about 60%, more preferably atleast about 75% and most preferably from about 75% to about 85%.

The process according to the invention first involves passing blackliquor into an evaporator train where it passes through one or moreevaporators. The liquid is then pumped into a high pressure pump wherethe pressure is increased so that the liquid does not boil when heated.The liquid is then fed into a heat exchanger and then into a heatedreactor for a specified amount of time. The reacted product is thencooled by a reactor feed and the flash gas is returned to theevaporators where it is condensed. The pressure is then reduced toatmospheric pressure and the product is fed into an agitated producttank. At this point several methods of additional processing areavailable.

For example, the reacted black liquor, after filtration, may be fired ina coal or bark boiler since the green liquor has been removed. Thisincreases the number of options for a recovery limited mill and likewiseincreases the pulping capacity of a mill, since more black liquor solidsdo not present a bottleneck at the recovery boiler.

The filtrate from the reacted liquor may be passed directly into thegreen liquor system and causticized using existing equipment andprocedures. The filtered char may be burned in a recovery or powerboiler.

Another treatment involves adding lime to the black liquor slurry in aslaker, with the resultant slurry being causticized and then filtered,thus producing white liquor directly from the black liquor. Thechar-lime-mud slurry may then be burned in a fluidized bed calcinerequipped to reclaim heat. This scenario eliminates the recovery boiler,evaporators, lime kiln and green liquor system.

The conversion of the black liquid into a filterable slurry through thisthermal treatment allows for several considerations:

1. Assuming that oxygen accounts for 35% of the mass of the organicfraction of the unreacted black liquor, and the oxygen is converted tocarbon dioxide using carbon present in the lignin structure, asignificant portion of the mass of the organic component of black liquorexits as carbon dioxide.

2. The conversion of complex water-soluble polymers (alkali lignins) tohydrophobic char allows the organic fraction of the reacted liquor to beseparated from the aqueous inorganic portion through conventionalfiltration methods. This eliminates the need for solids concentrationvia evaporation.

4. The viscosity of the reacted liquor, as compared to the viscosity ofthe unreacted black liquor, is reduced, preferably to a significantextent. A reactor product liquor at 80% solids will have approximatelythe same viscosity as unreacted black liquor at 60% solids.

Thus, the present invention has significant benefits over conventionalrecovery processes.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and features of the present invention will become apparentfrom the following detailed description considered in connection withthe accompanying drawings. It is to be understood, however, that thedrawings are designed as an illustration only and not as a definition ofthe limits of the invention.

In the drawings, wherein similar reference characters denote similarelements throughout the several views:

FIG. 1 shows a flow diagram of one embodiment of the recovery processaccording to the invention; and

FIG. 2 shows a flow diagram of a second embodiment according to theinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now in detail to the drawings and, in particular, FIG. 1 thereis shown a flow diagram depicting the inventive process for thermallytreating black liquor 1. The black liquor 1, which has approximately15-18% solids by mass is fed into an evaporator train comprising aplurality of evaporators 2. Typically, the desired concentration of theproduct from the evaporator train is approximately 30-50% solids.

At the outlet of evaporators 2, liquor 1 pumped by a high pressure pump3 to a pressure between 1200 psig and 2000 psig. This pressure isnecessary to prevent the liquid from boiling when is heated. From pump3, the liquid is transferred to a heat exchanger 4 where it is heated ata temperature of from, about 500° F. to about 575° F. with acorresponding pressure such that the heated liquor does not boil, and isthen transferred to a heated reactor 5 which is heated by some suitableheating means as for example steam. Reactor 5 to designed so that aresidence time of from 0 to 60 minutes is attained and is in the form ofa coiled tube, downflow tube, upflow tube or horizontal tube reactor. Agas vent (not shown) may also be used to remove buildup of productgases.

From reactor 5, the product is passed through the return side of heatexchanger 4 and then through a set of flash vessels 7. Any flash gas 6is returned to evaporators 2 where it is condensed with a condenser (notshown) and its heat reclaimed. The non-condensible gases contain someheat value when combusted and are used as fuel in the lime kiln orcalciner. The product exiting flash vessels 7 has approximately 40-70%solids and is at about 100 psig. The pressure is then reduced in apressure reduction valve 8 until the product is at atmospheric pressure.The product is then transferred to an agitated product tank 9.

At this point, the slurry produced from the above steps may be treatedin several different ways. For example, it may be pumped by pump 10 intoa settling tank 14. In settling tank 14, the suspended organic solidsfall to the bottom of the tank and the resulting green liquor solutionmay be drawn away for clarification.

The settled solids in settling tank 14 are pumped to a precoat filterrotary drum 17 to separate the organic solids from the green liquorfiltrate. The solids then flow from filter 17 to a liquor mix tank. Adilution line to the tank is controlled dependent on the desiredconcentration of solids to be pumped from mix tank 18 with pump 20 toheavy liquor tanks (not shown).

The filtrate from filter 17 is pumped via pump 16 to a microfiltrationunit 15 to remove any remaining suspended solids. Microfiltration unit15 can also be used to remove suspended solids from the green liquidsolution removed from settling tank 14. The filtrate frommicrofiltration unit 15 is then passed to the green liquor system.Solids collected in the microfiltration unit are passed into liquor mixtank 18.

Another way of treating the slurry from product tank 9 is to pump it viapump 10 into a second set of evaporators 13 until it is brought to thedesired solids concentration, where it is then transferred to heavyliquor tanks (not shown).

Yet another method of treating black liquor according to the inventionis shown in FIG. 2. In this embodiment, black liquor 1 proceeds throughthe same thermal treatment as discussed with respect to FIG. 1 until itreaches product tank 9. At this point, the slurry from product tank 9 ispumped via pump 25 into a slaker 26 where lime 36 is added. The grit anddregs are then removed from the slaker 26 with a screw feeder (notshown). Slurry from slaker 26 is then passed to a set of one or morecausticizers 27 to allow the sodium carbonate to be converted to sodiumhydroxide. From causticizer 27, the slurry is pumped via pump 28 to aclarifier 29, where white liquor is drawn off at a level above aclarification zone and pumped via pump 30 to a microfiltration unit 33.The white liquor filtrate obtained from microfiltration unit 33 isstored in a white liquor storage 34. Any solids from microfiltrationunit 33 are passed to a precoat filter 32 for further filtration.

The slurry collected at the bottom of clarifier 29 is pumped via pump 31to precoat filter 32. Other filtration devices could also be used. Thefiltrate from precoat filter 32 is then passed to microfiltration unit33. The filtered solids are subsequently burned to reclaim heat andconvert calcium carbonate to calcium oxide. This heat reclamation andconversion may be accomplished in a fluidized bed boiler, moving grateboiler, pressurized-fluidized-bed-combuster (PFBC), conventional rotarylime kiln or the slurry may be diluted somewhat and combusted in aconventional recovery boiler equipped for this alternative fuel.

The processes described above yield a significant benefit in therecovery of kraft chemicals. The following example illustrates theeffect that the inventive process has on black liquor.

EXAMPLE

A 100 lb sample of black liquor containing approximately 30% solids wastreated with the process described above, and its properties testedafter the black liquor was received in product tank 9. The properties ofthe black liquor before and after processing are set forth below inTable I.

Table I

    ______________________________________                                                Before       After                                                            reaction     reaction                                                 Component Mass (lb)                                                                              State     Mass (lb)                                                                             State                                    ______________________________________                                        Water     70.0     Aqueous   70      Aqueous                                  Carbon             10.56                                                                             Aqueous                                                                              9.40           Particulate                      Carbon Bound                                                                               4.14       Aqueous                                                                             1.03           Particulate                      Oxygen                                                                        Hydrogen Bound                                                                           1.17         Aqueous                                                                             1.17           Particulate                      Carbon                                                                        Sodium             0.00                                                                               n/a       0.00                                                                                     n/a                              Hydroxide                                                                     Sodium             9.54                                                                               Aqueous                                                                             9.54           Aqueous                          Carbonate                                                                     Sodium Sulfate                                                                           4.53         Aqueous                                                                             trace     Aqueous                               Sodium Sulfide                                                                           0.00         n/a       2.49                                                                                 Aqueous                              Carbon Dioxide                                                                           0.00         n/a       4.27                                                                                     Gas                              Oxygen             0.00                                                                               n/a       2.04                                                                                     Gas                              ______________________________________                                    

Residual sodium sulfate and sodium hydroxide were not factored into thecalculations and their presence would not affect the calculations.

As can be seen from the above table, several benefits are achieved withthe process according to the invention. Assuming that the reacted liquoris subsequently processed, the solids fired to a recovery boiler arereduced by 61.3%. In addition, the amount of water carried to the boilerwith the liquor is reduced by 82.1% and the volume of black liquor isreduced by 68.6%.

The process according to the invention greatly reduces the amount oforganic mass of black liquor. While it may appear that this reductionwould also reduce the heat of combustion, only a 3% reduction in theheat of combustion has been observed, most of this caused by the exitingof CO₂. This loss is greatly offset by the reduction in the evaporativeheat requirements of the water present and reduction in the heat lossdue to inorganic dead load when fired in a boiler.

Another benefit derived from the process according to the invention isthe densification of energy. A typical black liquor has a heat ofcombustion of about 6,500 BTU/lb. on a dry basis. After thermaltreatment and filtration according to the invention, the liquor slurrywill have a theoretical heat of combustion of 16,290 BTU/lb. The actualheat of combustion has been found to be approximately 85% of thetheoretical value, or 13,848 BTU/lb.

Accordingly, while only a few embodiments of the present invention havebeen shown and described, it is obvious that many changes andmodifications may be made thereunto without departing from the spiritand scope of the invention.

What is claimed is:
 1. A method of recovering pulping chemicals from aspent black liquor generated in a kraft pulping process, said spentblack liquor comprising organic materials having oxygen containingfunctional groups, said method comprising heating the spent black liquorfor a reaction time and at a reaction temperature and at a reactionpressure at which there is no or substantially no boiling of the spentblack liquor during said heating, such that all or a portion of theoxygen contained in said organic material is converted into carbondioxide to form a heated suspension comprising a microparticulate charsuspended in a green liquor liquid phase.
 2. The method according toclaim 1, whereas the step of heating comprises evaporating the blackliquor, pumping the liquor in a high pressure pump to a pressure atwhich there is no or substantially no boiling of said black liquor whenheated in a subsequent heating step, transferring the liquor to a heatexchanger, heating the black liquor to the elevated temperature to formheated liquor, transferring the heated black liquor to a reactor, andmaintaining the heated black liquor in the reactor for a reaction timesufficient for conversion of all or a portion of the oxygen into thecarbon dioxide.
 3. The method according to claim 2 further comprisingcooling the heated green liquor suspension to form a cooled suspension.4. The method according to claim 3, further comprising the step ofevaporating the cooled suspension until a desired suspended solids loadis achieved.
 5. The method according to claim 3, further comprising thesteps of:pumping the cooled suspension to a settling tank and allowingsuspended solids to fall to the bottom of the tank; drawing off thesupernatant green liquor from the settling tank; filtering the solidsfrom the settling tank; and combining any green liquor filtrate obtainedfrom said filtering step with said supernatant green liquor.
 6. Themethod according to claim 5, wherein the step of filtering is carriedout using a precoat filter.
 7. The method according to claim 5, whereinthe step of filtering is carried out using a microfiltration unit. 8.The method according to claim 5, wherein the step of filtering iscarried out using both a precoat filter and a microfiltration unit. 9.The method according to claim 3, further comprising the steps of:addinglime to said cooled green liquor suspension in a slaker to form amixture of green liquor suspension and lime; transferring green liquorsuspension and lime mixture from the slaker to a causticizer forconverting sodium carbonate to sodium hydroxide, wherein the greenliquor suspension is converted into a white liquor suspension withsolids; transferring the said white liquor and solids to a clarifier anddrawing off the white liquor; and filtering any remaining white liquorin the clarifier from the solids.
 10. The method according to claim 9,wherein the step of filtering is carried out using a microfiltrationunit.
 11. The method according to claim 9, wherein the step of filteringis carried out using a precoat filter.
 12. The method according to claim9, wherein the step of filtering is carried out using a microfiltrationunit and a precoat filter.
 13. The method according to claim 9, furthercomprising burning the solids.
 14. The method according to claim 3wherein said heated green liquor suspension is cooled by passing theheated green liquor suspension through one or more flash vessels,condensing any flash gas and reducing the pressure of the suspension toatmospheric pressure.
 15. The method according to claim 3, furthercomprising separating all or a portion of suspended solids in saidcooled green liquor suspension by sedimentation, filtration or acombination thereof.
 16. The method according to claim 2 wherein thereaction temperature is at least about 475° F.
 17. The method accordingto claim 16 wherein the reaction temperature is from about 475° F. toabout 625° F.
 18. The method according to claim 1 wherein the reactiontemperature is at least about 475° F.
 19. The method according to claim18 wherein the reaction temperature is from about 475° F. to about 625°F.
 20. The method according to claim 19 wherein the reaction temperatureis from about 500° F. to about 575° F.
 21. The method according to claim19 wherein the reaction pressure is from about 1200 psig to about 2000psig.
 22. The method according to claim 1 wherein at least about 50% ofthe oxygen is converted into the carbon dioxide.
 23. The methodaccording to claim 1 wherein at least about 50% of the oxygen isconverted into the carbon dioxide.